【Spring】Spring中8种常见依赖注入使用示例

在 Spring 中，IoC 注入可以通过多种方式实现，涵盖不同场景的依赖管理。以下是 8 种常见场景的详细示例及说明，结合 XML、注解和 Java 配置类三种方式。

1. 构造器注入（推荐方式）

通过构造器传递依赖，确保对象不可变且依赖完整。

注解方式：

@Service
public class OrderService {private final PaymentService paymentService;@Autowired // Spring 4.3+ 可省略（单构造器时）public OrderService(PaymentService paymentService) {this.paymentService = paymentService;}
}@Component
public class PaymentService {public void process() {System.out.println("Processing payment...");}
}

XML 方式：

<bean id="paymentService" class="com.example.PaymentService"/><bean id="orderService" class="com.example.OrderService"><constructor-arg ref="paymentService"/>
</bean>

Java 配置类：

@Configuration
public class AppConfig {@Beanpublic PaymentService paymentService() {return new PaymentService();}@Beanpublic OrderService orderService() {return new OrderService(paymentService());}
}

2. Setter 注入

通过 Setter 方法注入依赖，适合可选依赖或需要动态变更的场景。

注解方式：

@Service
public class ProductService {private InventoryService inventoryService;@Autowiredpublic void setInventoryService(InventoryService inventoryService) {this.inventoryService = inventoryService;}
}@Component
public class InventoryService {public void checkStock() {System.out.println("Checking stock...");}
}

XML 方式：

<bean id="inventoryService" class="com.example.InventoryService"/><bean id="productService" class="com.example.ProductService"><property name="inventoryService" ref="inventoryService"/>
</bean>

3. 字段注入（不推荐，但常见）

直接通过字段注入依赖，简洁但隐藏依赖关系。

@Service
public class CartService {@Autowiredprivate DiscountService discountService;
}@Component
public class DiscountService {public void applyDiscount() {System.out.println("Applying discount...");}
}

4. 集合注入（多个同类型 Bean）

注入多个相同接口/父类的实现类。

示例代码：

public interface NotificationSender {void send(String message);
}@Component
public class EmailSender implements NotificationSender {@Overridepublic void send(String message) {System.out.println("Email: " + message);}
}@Component
public class SmsSender implements NotificationSender {@Overridepublic void send(String message) {System.out.println("SMS: " + message);}
}

注入方式：

@Service
public class NotificationService {private final List<NotificationSender> senders;@Autowiredpublic NotificationService(List<NotificationSender> senders) {this.senders = senders;}public void broadcast(String message) {senders.forEach(sender -> sender.send(message));}
}

5. 条件注入（@Qualifier 或 @Primary）

解决多个同类型 Bean 的歧义性问题。

使用 `@Qualifier`：

public interface DataSource {void connect();
}@Component("mysqlDataSource")
public class MySqlDataSource implements DataSource {@Overridepublic void connect() {System.out.println("MySQL connected");}
}@Component("oracleDataSource")
public class OracleDataSource implements DataSource {@Overridepublic void connect() {System.out.println("Oracle connected");}
}@Service
public class ReportService {private final DataSource dataSource;@Autowiredpublic ReportService(@Qualifier("mysqlDataSource") DataSource dataSource) {this.dataSource = dataSource;}
}

使用 `@Primary`：

@Configuration
public class AppConfig {@Bean@Primary // 默认优先选择public DataSource mysqlDataSource() {return new MySqlDataSource();}@Beanpublic DataSource oracleDataSource() {return new OracleDataSource();}
}

6. 外部化配置注入（@Value）

注入配置文件（如 application.properties）中的值。

配置文件：

app.name=MySpringApp
app.max.connections=10

注入方式：

@Component
public class AppConfigBean {@Value("${app.name}")private String appName;@Value("${app.max.connections}")private int maxConnections;// Getter/Setter
}

7. 循环依赖处理

构造器注入无法解决循环依赖，需改用 Setter 注入。

示例代码：

@Service
public class ServiceA {private ServiceB serviceB;@Autowiredpublic void setServiceB(ServiceB serviceB) {this.serviceB = serviceB;}
}@Service
public class ServiceB {private ServiceA serviceA;@Autowiredpublic void setServiceA(ServiceA serviceA) {this.serviceA = serviceA;}
}

8. 条件化 Bean 注入（@Profile 或 @Conditional）

根据环境或条件动态选择 Bean。

使用 `@Profile`：

@Configuration
public class DataSourceConfig {@Bean@Profile("dev")public DataSource devDataSource() {return new H2DataSource();}@Bean@Profile("prod")public DataSource prodDataSource() {return new MySqlDataSource();}
}

使用 `@Conditional`：

public class MongoDBCondition implements Condition {@Overridepublic boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {return "true".equals(context.getEnvironment().getProperty("enable.mongo"));}
}@Configuration
public class DatabaseConfig {@Bean@Conditional(MongoDBCondition.class)public DataSource mongoDataSource() {return new MongoDataSource();}
}